I just think you overstress yourself. When you concentrate 100% on your serve and serve goes well you tend to relax just after it. Why? Because you want it to go in so badly....Your brain reacts to overpressure....And opposite, when groundstrokes start to fell you concentrate 100% on strokes and tend to relax after wininng the point (thus on your serve...). My advice , try to relax yourself. Don't overdo, setup your mind to do everything 75% ( 3/4 pace on every shot: serve, groundstroke, volley...Tend to relax and enjoy the game, stop going 100% on every shot. Just get to the ball and play it back, under control. You don't have to hit winners every time , but stop making unforced errors. Let him play the ball , let him worry how to make an extra shot instead of beating yourself by unforced errors.